Molecular Fingerprint Region Research Articles

Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Cavity-enhanced dual-comb spectroscopy in the molecular fingerprint region using free-running quantum cascade lasers

Ultra-broadband spectroscopy using a 2–11.5 µm IDFG-based supercontinuum source

Supercontinuum sources based on intrapulse difference frequency generation (IDFG) from mode-locked lasers open new opportunities in mid-infrared gas spectroscopy. These sources provide high power and ultra-broadband spectral coverage in the molecular fingerprint region with very low relative intensity noise. Here, we demonstrate the performance of such a light source in combination with a multipass cell and a custom-built Fourier transform spectrometer (FTS) for multispecies trace gas detection. The light source provides a low-noise, ultra-broad spectrum from 2–11.5 µm with ∼3 W output power, outperforming existing mid-infrared supercontinuum sources in terms of noise, spectral coverage, and output power. This translates to an excellent match for spectroscopic applications, establishing (sub-)ppb sensitivity for molecular hydrocarbons (e.g., CH4, C2H4), oxides (e.g., SO2, NOx), and small organic molecules (e.g., acetone, ethyl acetate) over the spectral range of the supercontinuum source with a measurement time varying from seconds to minutes. We demonstrate a practical application by measuring the off-gas composition of a bioreactor containing an acidic ammonia-oxidizing culture with the simultaneous detection of multiple nitrogen oxides (NO, NO2, N2O, etc.). As the different species absorb various parts of the spectrum, these results highlight the functionality of this spectroscopic system for biological and environmental applications.

Design and numerical modeling of chalcogenide parabolic‐core waveguide for on‐chip supercontinuum generation extending from near‐IR region to mid‐IR region

AbstractWe present the design and computational examination of a waveguide structure with a parabolic core, implemented in AsSe2 chalcogenide glass. This structure aims to generate a mid‐infrared (mid‐IR) supercontinuum (SC) spectrum, covering the wavelength range of 1.4−7.6 µm. It includes molecular fingerprint region as well as earth's atmospheric transparent window. A pulsed laser providing pulses of peak power of 250 W and pulse width of 200 fs at 2 µm was considered as the pump. The results illustrate that the broadband SC spectrum can be achieved in a short (~8 mm) waveguide pumped with commercially available fiber laser at 2 µm. The novelty of this work lies in the generation of a mid‐IR SC spectrum using a parabolic‐core waveguide pumped in the normal dispersion region, which has not been investigated earlier. This design is crucial as the fabricated rectangular waveguide does not remain rectangular after laser postprocessing but becomes similar to the proposed parabolic design.

Broadband Coherent Raman Scattering Spectroscopy at 50,000,000 Spectra per Second

Raman scattering spectroscopy is widely used as an analytical technique in various fields, but its measurement process tends to be slow due to the low scattering cross-section. In the last decade, various broadband coherent Raman scattering spectroscopy techniques have been developed to address this limitation, achieving a measurement rate of 500 kSpectra/s. Here, we present a substantially increased measurement rate of 50 MSpectra/s, which is 100 times higher than the previous state-of-the-art, by developing time-stretch coherent Raman scattering spectroscopy. Our newly developed system, based on a mode-locked Yb fiber laser, enables highly efficient broadband excitation of molecular vibrations via impulsive stimulated Raman scattering with an ultrashort femtosecond pulse and sensitive time-stretch detection with a picosecond probe pulse at a high repetition rate of the laser. As a proof-of-concept demonstration, we measure broadband coherent Stokes Raman scattering spectra of organic compounds covering the molecular fingerprint region from 200 to 1,200 cm −1 . This high-speed broadband vibrational spectroscopy technique holds promise for unprecedented measurements of sub-microsecond dynamics of irreversible phenomena and extremely high-throughput measurements.

Broadband Up‐Conversion Mid‐Infrared Time‐Stretch Spectroscopy

AbstractTime‐stretch spectroscopy is a powerful tool for studying transient and nonrepetitive physical events. Its extension to the fundamental molecular fingerprint region is promising for chemical and biological research but encounters difficulties in mid‐infrared photodetection and strong attenuation of aqueous environments. Here, mid‐infrared time‐stretch spectroscopy via coincidently‐pumped nonlinear frequency conversion for efficient mid‐infrared generation and detection is demonstrated. Broadband (≈200 cm−1) mid‐infrared spectra at 77.5 Mspectra s−1 with high signal‐to‐noise ratio are recorded. The single‐shot signal‐to‐noise ratio (SNR) measured within 2 ns exceeds 200. The method reveals fast spectral dynamics in a laser‐induced liquid evaporation process with a temporal resolution of 12.9 ns. Furthermore, probing transient spectra during evaporation opens up a new opportunity for interrogating chemicals inside or behind a thick water layer. Integrating high speed, broad bandwidth, high SNR, and the potential for operating under an aqueous environment, this method will benefit many applications including molecular dynamics, chemical reactions, and biological diagnostics.

In-line synthesis of multi-octave phase-stable infrared light.

Parametric downconversion driven by modern, high-power sources of 10-fs-scale near-infrared pulses, in particular intrapulse difference-frequency generation (IPDFG), affords combinations of properties desirable for molecular vibrational spectroscopy in the mid-infrared range: broad spectral coverage, high brilliance, and spatial and temporal coherence. Yet, unifying these in a robust and compact radiation source has remained a key challenge. Here, we address this need by employing IPDFG in a multi-crystal in-line geometry, driven by the 100-W-level, 10.6-fs pulses of a 10.6-MHz-repetition-rate, nonlinearly post-compressed Yb:YAG thin-disk oscillator. Polarization tailoring of the driving pulses using a bichromatic waveplate is followed by a sequence of two crystals, LiIO3 and LiGaS2, resulting in the simultaneous coverage of the 800-cm-1-to-3000-cm-1 spectral range (at -30-dB intensity) with 130 mW of average power. We demonstrate that optical-phase coherence is maintained in this in-line geometry, in theory and experiment, the latter employing ultra-broadband electro-optic sampling. These results pave the way toward coherent spectroscopy schemes like field-resolved and frequency-comb spectroscopy, as well as nonlinear, ultrafast spectroscopy and optical-waveform synthesis across the entire infrared molecular fingerprint region.

The ro-vibrational ν2 mode spectrum of methane investigated by ultrabroadband coherent Raman spectroscopy.

We present the first experimental application of coherent Raman spectroscopy (CRS) on the ro-vibrational ν2 mode spectrum of methane (CH4). Ultrabroadband femtosecond/picosecond (fs/ps) CRS is performed in the molecular fingerprint region from 1100 to 2000cm-1, employing fs laser-induced filamentation as the supercontinuum generation mechanism to provide the ultrabroadband excitation pulses. We introduce a time-domain model of the CH4 ν2 CRS spectrum, including all five ro-vibrational branches allowed by the selection rules Δv = 1, ΔJ = 0, ±1, ±2; the model includes collisional linewidths, computed according to a modified exponential gap scaling law and validated experimentally. The use of ultrabroadband CRS for in situ monitoring of the CH4 chemistry is demonstrated in a laboratory CH4/air diffusion flame: CRS measurements in the fingerprint region, performed across the laminar flame front, allow the simultaneous detection of molecular oxygen (O2), carbon dioxide (CO2), and molecular hydrogen (H2), along with CH4. Fundamental physicochemical processes, such as H2 production via CH4 pyrolysis, are observed through the Raman spectra of these chemical species. In addition, we demonstrate ro-vibrational CH4 v2 CRS thermometry, and we validate it against CO2 CRS measurements. The present technique offers an interesting diagnostics approach to in situ measurement of CH4-rich environments, e.g., in plasma reactors for CH4 pyrolysis and H2 production.

High-resolution molecular fingerprinting in the 11.6-15 µm range by a quasi-CW difference-frequency-generation laser source.

We report an approach for high-resolution spectroscopy using a widely tunable laser emitting in the molecular fingerprint region. The laser is based on difference-frequency generation (DFG) in a nonlinear orientation-patterned GaAs crystal. The signal laser, a CO2 gas laser, is operated in a kHz-pulsed mode while the pump laser, an external-cavity quantum cascade laser, is finely mode-hop-free tuned. The idler radiation covers a spectral range of ∼11.6-15 µm with a laser linewidth of ∼ 2.3 MHz. We showcase the versatility and the potential for molecular fingerprinting of the developed DFG laser source by resolving the absorption features of a mixture of several species in the long-wavelength mid-infrared. Furthermore, exploiting the wide tunability and resolution of the spectrometer, we resolve the broadband absorption spectrum of ethylene (C2H4) over ∼13-14.2 µm and quantify the self-broadening coefficients of some selected spectral lines.

All-fiber Mid-infrared enhanced supercontinuum generation in an Erbium-doped ZBLAN fiber amplifier

We demonstrated all-fiber mid-infrared enhanced supercontinuum generation in an Erbium-doped ZBLAN fiber amplifier. A 2 μm low repetition rate noise like-pulse mode-locked fiber laser was adopted as the pulse seed for the system. After amplified in a Thulium-doped fiber amplifier, the noise like-pulse generated a supercontinuum covering the range from ∼2 μm to ∼3.8 μm in a piece of InF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> fiber. Then, the supercontinuum was injected into an Erbium-doped ZBLAN fiber amplifier as the seed for the next stage in-amplifier mid-infrared enhanced supercontinuum generation. A novel silica-fluoride fiber pump combiner was designed and fabricated to couple the 976 nm LD pump laser into the Erbium-doped ZBLAN fiber. The supercontinuum laser source realized mid-infrared enhancement and power boosting beyond 2.7 μm in the Erbium-doped ZBLAN fiber amplifier. A ∼1.4 W mid-infrared enhanced SC with long wavelength edge of ∼4200 nm and 97% power beyond 2.7 μm was obtained. This simple and compact all-fiber supercontinuum laser source has a great potential application in the 3-5 μm molecular fingerprint region.

Intense infrared lasers for strong-field science

The advent of chirped-pulse amplification in the 1980s and femtosecond Ti:sapphire lasers in the 1990s enabled transformative advances in intense laser–matter interaction physics. Whereas most of experiments have been conducted in the limited near-infrared range of 0.8–1 μm, theories predict that many physical phenomena such as high harmonic generation in gases favor long laser wavelengths in terms of extending the high-energy cutoff. Significant progress has been made in developing few-cycle, carrier-envelope phase-stabilized, high-peak-power lasers in the 1.6–2 μm range that has laid the foundation for attosecond X ray sources in the water window. Even longer wavelength lasers are becoming available that are suitable to study light filamentation, high harmonic generation, and laser–plasma interaction in the relativistic regime. Long-wavelength lasers are suitable for sub-bandgap strong-field excitation of a wide range of solid materials, including semiconductors. In the strong-field limit, bulk crystals also produce high-order harmonics. In this review, we first introduce several important wavelength scaling laws in strong-field physics, then describe recent breakthroughs in short- (1.4–3 μm), mid- (3–8 μm), and long-wave (8–15 μm) infrared laser technology, and finally provide examples of strong-field applications of these novel lasers. Some of the broadband ultrafast infrared lasers will have profound effects on medicine, environmental protection, and national defense, because their wavelengths cover the water absorption band, the molecular fingerprint region, as well as the atmospheric infrared transparent window.

Multi-Modal Stand-Off Detection of Plastics for Sorting Application

Plastics are extremely valuable to our society and are widely used in many industries such as food, beverage, textile, healthcare, automotive, aerospace, construction, agriculture, oil, etc. Since plastics do not degrade easily with time, the discarded plastics pose a grave environmental problem. Only less than 10% of plastics is recycled in the US. Therefore, there is an urgent need to increase the recycling of plastic products. At present, sorting of plastic waste is carried out manually which is time consuming, costly and manual labor intensive. To address this problem, a novel multi-modal sensor platform capable of improving the accuracy of automated sorting of various plastics by using molecular identification has been developed. This multimodal technique of standoff photothermal-photoacoustic detection is a combination of mid-infrared (MIR) photo-thermal spectroscopy and photoacoustic spectroscopy for characterizing surfaces. Since MIR is free of overtones, this region is known as the molecular fingerprint region. In this technique, the plastic surface is illuminated with a pulsed laser beam from a tunable quantum cascade laser (QCL). The photothermal detection is accomplished by collecting the scattered light from the plastic surface which contains the molecular vibration spectrum of the sample. A silicon bi-material microcantilever, fabricated by depositing a thin metal film on one of its sides is used as a sensitive uncooled infrared detector. Monitoring the bi-material cantilever bending as a function of illumination wavelength provides the IR absorption spectrum of the plastic material. Simultaneously, the photoacoustic signal generated by the absorption of light is monitored by a sensitive microphone. A highly sensitive microfabricated acoustic resonator is used for collecting the photoacoustic signal. Monitoring the microphone response as a function of wavelength produces photoacoustic spectrum (PAS) of the sample. This non-destructive characterization technique will help in enhancing the quality of plastic sorting process at materials recovery facilities and other industries by making the process inexpensive and time efficient.

Mid-infrared electronic wavelength tuning through intracavity difference-frequency mixing in Cr:ZnSe lasers

Mid-infrared tunable coherent light sources are used in various laser applications, such as trace gas detection, laser processing, and biomedical diagnostics. This study demonstrates mid-infrared generation in the 8.3–11 µm (i.e., 900–1200 cm−1) spectral range by configuring intracavity difference-frequency generation (DFG) using ZnGeP2 (ZGP) in an electronically tuned Cr:ZnSe laser. The broad tunability is achieved with the maximum pulse energies exceeding 100 μJ by combining the electronic wavelength tuning with sligh angle adjustments (Δθ < 0.5°) of ZGP under the spectral noncritical phase-matching condition of the nonlinear material. The proposed DFG method is generalized to give access to a significant fraction of the molecular fingerprint region by utilizing selenide compounds (e.g., AgGaSe2, CdSe) in addition to ZGP, revealing the remarkable potential of ultrabroadband electronic mid-infrared scanning for numerous spectroscopic applications.

Experimental realization of scanning quantum microscopy

Quantum imaging is an ever expanding research field in which the aim is to exploit the quantum nature of light to enhance image reconstruction capabilities. Despite a number of successful demonstrations for quantum imaging, quantum microscopy still seems out of the range for practical applications due to different physical and technical reasons. Here, we propose an imaging method exploiting the quantum correlations of photon pairs and a scanning microscope to achieve fast, single mode quantum imaging. We first test our technique on a metal grating to estimate the resolution capabilities of our system. Moreover, we assess its potential in terms of the number of available independent pixels at full resolution compared to different quantum imaging approaches. Finally, we demonstrate scanning quantum microscopy of onion epithelial cells, paving the way toward scalable quantum microscopy for bio-physical applications. Our results, combined with the rapidly evolving photon-pair generation and detection technology toward the mid-infrared, could lead to an extension of quantum microscopy applications toward the mid-infrared to access the molecular fingerprint region.

Simple approach to broadband mid-infrared pulse generation with a mode-locked Yb-doped fiber laser.

Broadband mid-infrared (MIR) molecular spectroscopy demands a bright and broadband light source in the molecular fingerprint region. To this end, intra-pulse difference frequency generation (IDFG) has shown excellent properties among various techniques. Although IDFG systems pumped with 1.5- or 2-µm ultrashort pulsed lasers have been extensively developed, few systems have been demonstrated with 1-µm lasers, which use bulky 100-W-class high-power Yb thin-disk lasers. In this work, we demonstrate a simple and robust approach of 1-µm-pumped broadband IDFG with a conventional mode-locked Yb-doped fiber laser. We first generate 3.3-W, 12.1-fs ultrashort pulses at 50 MHz by a simple combination of spectral broadening with a short single-mode fiber and pulse compression with chirped mirrors. Then, we use them for pumping a thin orientation-patterned gallium phosphide crystal, generating 1.2-mW broadband MIR pulses with the -20-dB bandwidth of 480 cm-1 in the fingerprint region (760-1240 cm-1, 8.1-13.1 µm). The 1-µm-based IDFG system allows for additional generations of ultrashort pulses in the ultraviolet and visible regions, enabling, for example, 50-MHz-level high-repetition-rate vibrational sum-frequency generation spectroscopy or pump-probe spectroscopy.

Efficient generation of few-cycle pulses beyond 10\xa0\u03bcm from an optical parametric amplifier pumped by a 1-\xb5m laser system

Nonlinear vibrational spectroscopy profits from broadband sources emitting in the molecular fingerprint region. Yet, broadband lasers operating at wavelengths above 7 μm have been lacking, while traditional cascaded parametric frequency down-conversion schemes suffer from exceedingly low conversion efficiencies. Here we present efficient, direct frequency down-conversion of femtosecond 100-kHz, 1.03-μm pulses to the mid-infrared from 7.5 to 13.3 μm in a supercontinuum-seeded, tunable, single-stage optical parametric amplifier based on the wide-bandgap material Cd0.65Hg0.35Ga2S4. The amplifier delivers near transform-limited, few-cycle pulses with an average power > 30 mW at center wavelengths between 8.8 and 10.6 μm, at conversion efficiencies far surpassing that of optical parametric amplification followed by difference-frequency generation or intrapulse difference-frequency generation. The pulse duration at 10.6 μm is 101 fs corresponding to 2.9 optical cycles with a spectral coverage of 760–1160 cm−1. CdxHg1−xGa2S4 is an attractive alternative to LiGaS2 and BaGa4S7 in small-scale, Yb-laser-pumped, few-cycle mid-infrared optical parametric amplifiers and offers a much higher nonlinear figure of merit compared to those materials. Leveraging the inherent spatial variation of composition in CdxHg1−xGa2S4, an approach is proposed to give access to a significant fraction of the molecular fingerprint region using a single crystal at a fixed phase matching angle.

Dissipative Kerr solitons in semiconductor ring lasers

Dissipative Kerr solitons are self-organized optical waves arising from the interplay between Kerr effect and dispersion. They can form spontaneously in nonlinear microresonators pumped with an external continuous-wave laser, which provides the parametric gain for the proliferation of an ultrastable frequency comb. These miniaturized and battery driven microcombs have become a disruptive technology for precision metrology, broadband telecommunication and ultrafast optical ranging. In this work, we report on the first experimental observation of dissipative Kerr solitons generated in a ring cavity with a fast semiconductor gain medium. The moderate quality factor of the ring cavity is compensated by the giant resonant Kerr nonlinearity of a quantum cascade laser, which is more than a million times larger than in Si3N4. By engineering the dispersion of the cavity, we observe the formation of bright dissipative Kerr solitons in the mid-infrared range. Soliton formation appears after an abrupt symmetry breaking between the two lasing directions of the ring cavity. The pump field of the soliton is generated by direct electrical driving and closely resembles the soliton Cherenkov radiation observed in passive microcombs. Two independent techniques shed light on the waveform and coherence of the solitons and confirm a pulse width of approximately 3 ps. Our results extend the spectral range of soliton microcombs to mid-infrared wavelengths and will lead to integrated, battery driven and turnkey spectrometers in the molecular fingerprint region.

Phonon engineering of boron nitride via isotopic enrichment

Phonon polaritons (PhPs) enable a variety of applications, yet it requires PhPs supported in the desired frequency range. The two BN allotropes (cubic and hexagonal, cBN and hBN) are of particular interest, as their optic phonons fall within the so-called molecular-fingerprint region (~ 1000–1610 cm−1). However, there remains a spectral gap between PhPs covered by these two, limiting applications. Thus, we isotopically engineered hBN and cBN and examined the optic phonons. For hBN, enhancement of the optic phonon lifetimes and shifted frequencies are observed. However, lifetimes are observed to decrease with the enrichment of cBN by 10B, 11B, and 15N. We propose that the reduced lifetimes are not due to intrinsic loss, but rather increased defect concentrations resulting from the modified growth, supported by first-principles calculations. Thus, reducing the extrinsic defects in isotopically engineered cBN may present a path toward overcoming these restrictions for applications in the molecular-fingerprint region.

Fabrication of porous chitosan particles using a novel two-step porogen leaching and lyophilization method with the label-free multivariate spectral assessment of live adhered cells

Porous chitosan (CS) particles were fabricated using a novel two-step technique that employed a porogen leaching phase followed by lyophilization or freeze-drying. Poly(ethylene glycol) (PEG) was mixed as a porogen in two different quantities with the CS solution before particle synthesis via coacervation. After the PEG leached out into deionized (DI) water at an elevated constant temperature, the final freeze-dried CS particles revealed surface features that resembled pore pockets. A three-dimensional (3D) culture of murine osteoblast cell line (OB-6) was seeded on these particles to analyze the effect of the porous structure on the cell activity, as compared to a control group with no added porogen. The results highlighted an enhancement in cell adhesion and proliferation on the two porous sample groups. A Raman spectroscopy-based label-free technique for live cell biomarker analysis was applied using multivariate spectral analysis. Results of the spectral analysis in the molecular fingerprint region corresponding to the Raman shift between 900 cm−1 and 1700 cm−1inferred inter-group variations. The bands at 1005 cm−1 and 1375 cm−1 were assigned to the live cell biomarkers phenylalanine and glycosaminoglycan, respectively, and were assessed during the multivariate spectral analysis. The corresponding score plot and loading information generated from the Principal Component Analysis (PCA) of the Raman spectrum at day 7 and day 14, pointed at inter-group spectral variations related to cell adhesion and proliferation between the two porous CS particle groups and the control.

Design of mid-infrared filter array based on plasmonic metal nanodiscs array and its application to on-chip spectrometer

Mid-infrared wavelengths are called the molecular fingerprint region because it contains the fundamental vibrational modes inherent to the substances of interest. Since the mid-infrared spectrum can provide non-destructive identification and quantitative analysis of unknown substances, miniaturized mid-infrared spectrometers for on-site diagnosis have attained great concern. Filter-array based on-chip spectrometer has been regarded as a promising alternative. In this study, we explore a way of applying a pillar-type plasmonic nanodiscs array, which is advantageous not only for excellent tunability of resonance wavelength but also for 2-dimensional integration through a single layer process, to the multispectral filter array for the on-chip spectrometer. We theoretically and experimentally investigated the optical properties of multi-periodic triangular lattices of metal nanodiscs array that act as stopband filters in the mid-infrared region. Soft-mold reverse nanoimprint lithography with a subsequent lift-off process was employed to fabricate the multispectral filter array and its filter function was successfully extracted using a Fourier transform infrared microscope. With the measured filter function, we tested the feasibility of target spectrum reconstruction using a Tikhonov regularization method for an ill-posed linear problem and evaluated its applicability to the infrared spectroscopic sensor that monitors an oil condition. These results not only verify that the multispectral filter array composed of stopband filters based on the metal nanodiscs array when combined with the spectrum reconstruction technique, has great potential for use to a miniaturized mid-infrared on-chip spectrometer, but also provide effective guidance for the filter design.

1.7–18 µm mid-infrared supercontinuum generation in a dispersion-engineered step-index chalcogenide fiber

We demonstrate 1.7–18 µm mid-infrared coherent supercontinuum generation by means of the full transmission window of a unique dispersion-engineered step-index Ge-Se-Te fiber. Our study thus opens the entire molecular fingerprint region to future chalcogenide fiber platforms.